1. Field of the Invention
This invention relates to a method for curing organosiloxane compositions. More particularly, this invention relates to a method for curing organosiloxane compositions by a platinum group metal catalyzed hydrosilation reaction in the presence of cure-inhibiting impurities such as carboxylic acids. The present method is particularly useful for curing gels used as protective coatings for electronic components in the presence of "no-clean" solder fluxes containing carboxylic acids.
2. Background Information
Organosiloxane materials elastomers resins and gels exhibit a number of physical, chemical and electrical properties that make them desirable for use as coatings, encapsulants and potting materials for electronic and electrical components.
In typical applications, curable organosiloxane compositions are applied to substrates such as printed circuit boards to which electronic components such as transistors, integrated circuits and other solid state devices have been attached by soldering. The organosiloxane composition is then cured. In many applications gels are more desirable than elastomeric or resinous materials based on their low modulus, which allows these materials to expand in response to stresses resulting from heat-induced expansion of electronic components. If not relieved these stresses are sufficient to cause breakage of delicate wire bonds and other parts, resulting in malfunction of the component.
Organosiloxane compositions that cure by a hydrosilation reaction are preferred for protection of electrical and electronic circuits because they cure at relatively low temperatures and do not generate by-products during curing. The curing reaction involves alkenyl radicals such as vinyl and silicon-bonded hydrogen atoms.
One disadvantage of organosiloxane compositions that cure by the reaction of silicon-bonded hydrogen atoms is the ability of silicon-bonded hydrogen atoms to react with water and organic compounds containing hydroxyl groups. These reactions reduce the number of silicon-bonded hydrogen atoms available to cure the organosiloxane composition. In particular, it has been difficult to completely cure organosiloxane compositions using a hydrosilation reaction or the reaction between silicon bonded hydrogen atoms and silanol (.tbd.SiOH) groups in the presence of organic compounds such as alcohols and carboxylic acids.
A recent trend in the electronics industry has been to eliminate the organic solvents formerly used to remove fluxes that facilitate soldering of components to printed circuit boards. These fluxes contain relative large amounts of carboxylic acids. The present inventors discovered that the acids present in the flux react with a portion of the silicon-bonded hydrogen atoms present in the curable composition, thereby reducing the amount of organohydrogensiloxane available for curing the organosiloxane composition. Because the organohydrogensiloxane is distributed throughout the curable organosiloxane composition, it is difficult to estimate the amount of organohydrogensiloxane that will react with the solder flux. Adding an excess of the organohydrogensiloxane to the curable composition to compensate for the amount that will react can result in an excessive degree of crosslinking and a cured product that is too hard or brittle if not all the excess organohydrogensiloxane reacts with the solder flux.
An preferred objective of this invention is to provide a method for curing organosiloxane compositions in the presence of "no-clean" solder fluxes, where the curing agent is an organohydrogensiloxane.